It doesn't appear that you can. The mkqnx6fsimg program "produces a binary image file containing a Power-Safe (fs-qnx6.
so) filesystem" (from the 660 docs).

To actually make a bootable QNX6 partition, you need the mkqnx6fs program, which is only available for QNX6 hosts. This 
is how we have created bootable QNX6 partitions in the past (on a 6.6 host where the target disk is /dev/hd1 and /fs is 
available to mount the partition):

# fdisk /dev/hd1 delete -a
# fdisk add -s1 -t177
# fdisk boot -s1
# dloader -H /dev/hd1 pc1
# dloader -H /dev/hd1t177 pc2
# mkqnx6fs /dev/hd1t177
# mount -t qnx6 /dev/hd1t177 /fs
# cp -V /path/to/the/ifs/file.ifs /fs/.boot/boot_smp.ifs

Then copy over files to /fs (which gets mounted as / on the actual target) to make it usable. This is probably where you
 could use the output of mkqnx6fsimg, although we usually just copy over files directly.
While this works, the process, admittedly, feels clunky. Would love to know if anyone knows a better way to do this.

In examining the output of the mkqnx6fsimg, it appears that the utility is including the boot loader.

Also if you map /.boot/some_image = some_image.img  it doesn't complain.

However, I am having trouble getting the file system to mount...

Kevin